How do I assign a private IP to a user?

Before explaining how to assign a private IP to a user, it's important to note the impact of doing so.

Each reseller is given a limited number of private IPs (check your plan details for more information). Unless you know you will indefintely be working with a certain number of users, you will most likely not want to use these up too quickly, as deleting a user with a private IP will result in permanently losing it. Because of this, it's best to only assign private IPs to larger clients whom you expect to work with for the indefinite future. Users without a private IP will share the IP address of your main administrative account.

It's important to note that new, private IPs should be warmed up by initiall sending smaller blasts (under 10,000 emails per day) until they have a chance to build up a reputation.